This invention relates to a radio receiver arranged to automatically select a system for maintaining the optimum tuning of the receiver in accordance with the signal receiving condition of the receiver.
As is well known, in order to maintain the tuning of a radio receiver an automatic frequency control (AFC) system is used in many radio receivers. The AFC system is arranged such that, when a tuning error of the receiver is detected in accordance with an output signal of an intermediate amplifier or demodulator an output frequency of a local oscillator is controlled by a negative feedback thereto of a control voltage whose magnitude varies with an amount of tuning error so as to stabilize the local oscillator frequency.
The above AFC system properly functions when the receiver receives a strong broadcast signal. However, it is difficult for such an AFC system to properly function when a weak broadcast signal is received because of difficulty of accurate detection of tuning error. As a result, the local oscillator frequency tends to drift, making the maintenance of tuning of receiver difficult. Particularly, if there is a strong broadcast station or an interference wave in the adjacent frequency range the AFC system fails to lock the local oscillator frequency. In the case of a car radio receiver, the AFC system does not properly operate because the strength of a received broadcast signal always changes.
Radio receivers are known which use a frequency synthesizer tuner using a phase locked loop (PLL) having a voltage-controlled oscillator (VCO) which acts as a local oscillator. With a radio receiver using the phase locked loop the local oscillator frequency can be locked irrespective of the strength of a received broadcast signal. On the other hand, the frequency synthesizer tuner suffers from a disadvantage that since noises appearing at the output of a phase comparator incorporated in the phase locked loop are superimposed on the local oscillator signal the signal-to-noise ratio and the selectivity of the tuner are lowered. Further, with the PLL frequency synthesizer tuner, since the local oscillator frequency is caused to vary stepwise, e.g., in 100 KHz or 200 KHz steps, it is difficult to accurately adjust the intermediate frequency of tuner to the center frequency of an intermediate frequency filter used in the tuner. Accordingly, in practice, many frequency synthesizer tuners are not set to the optimum point of the sensitivity and distortion characteristics.